Wöhler curves relate directly to specific test specimens and environments. Test specimens for assessment of the fatigue properties of materials are often small unnotched peaces of material with smooth surfaces. Thus, the corresponding Wöhler curve represents something that is very dissimilar, and therefore not immediately pertinent to many structural elements in real engineering applications.

One important concept in expanding the applicability domain of fatigue test data is the Similarity Principle which implies that high cycle fatigue at a local surface is governed by the local stress field. Consequently, according to the similarity principle, the Wöhler curve of an unnotched material can be directly applied to a notched specimen of the same material, if the analysis is based on the local stress field of the considered regions. For instance.